Magnesium is an attractive material for application in motor vehicles because it is both a strong and lightweight material. The use of magnesium in motor vehicles is not new. Race driver Tommy Milton won the Indianapolis 500 in 1921 driving a car with magnesium pistons. A few years after that magnesium pistons entered mainstream automotive production. By the late 1930's over 4 million magnesium pistons had been produced. Even in the early days of car production, the weight-to-strength ratio of magnesium, compared with other commonly-used materials, was well-known.
Considering the recent increase in fuel prices driven largely by increased global demand, more attention is being given to any practical and economically viable step that can be taken to reduce vehicle weight without compromising strength and safety. Accordingly, magnesium is increasingly becoming an attractive alternative to steel, aluminum and polymers, given its ability to simultaneously meet crash-energy absorbing requirements while reducing the weight of vehicle components. Having a density of 1.8 kg/L, magnesium is 36% lighter per unit volume than aluminum (density=2.70 kg/L) and is 78% lighter per unit volume than steel (density=7.70 kg/L). Magnesium alloys also hold a competitive weight advantage over polymerized materials, being 20% lighter than most conventional glass reinforced polymer composites.
Beyond pistons, numerous other vehicle components are good candidates for being formed from magnesium, such as inner door panels, dashboard supports and instrument panel support beams. In the near-term it is anticipated that components made from magnesium for high volume use in the motor vehicle might also include powertrain, suspension and chassis components.
The fact that the surface “skin” of die-cast magnesium has better mechanical properties over the bulk than more commonly used materials, thinner (ribbed) and lighter die-castings of magnesium can be produced to meet their functional requirements. Such components can have sufficiently high strength per unit area to compete with more common and heavier aluminum and plastic components. Furthermore, magnesium has considerable manufacturing advantages over other die-cast metals, such as aluminum, being able to be cast closer to near net-shape thereby reducing the amount of material and associated costs. Particularly, components can be routinely cast at 1.0 mm to 1.5 mm wall thickness and 1 to 2 degree draft angles, which are typically ½ that of aluminum. The extensive fluid flow characteristics of magnesium offers a single, large casting to replace a plurality of steel fabrications. Magnesium also has a lower latent heat and reduced tendency for die pick-up and erosion. This allows a reduced die-casting machine cycle time (˜25% higher productivity) and 2 to 4 times longer die life (from 150-200,000 to 300-700,000 shots) compared with that of aluminum casting.
However, the use of magnesium in automotive components is burdened with certain drawbacks. While magnesium is abundant as a natural element, it is not available at a level to support automotive volumes. This situation causes hesitation among engineers to design and incorporate magnesium components. On the occasion when the magnesium is selected as the material of choice, designers fail to integrate die-casting design with manufacturing feasibility in which the mechanical properties, filling parameters, and solidification profiles are integrated to predict casting porosity and property distribution.
The raw material cost of magnesium relative to other commonly used materials is also an impediment to mass implementation in the automotive industry. Current techniques for casting parts from magnesium make expanding the use of magnesium into a broader array of products less attractive. Presently, all large die-castings are produced in high pressure, cold-chamber machines where the metal is injected from one central location. This approach results in inferior material properties and waste material.
Accordingly, in order to make the use of magnesium in the production of vehicle components more attractive to manufacturers, a new approach to product casting is needed. This new approach is the focus of the present invention.